Exhaust gases may be released as a result of combustion (e.g., of fossil fuels or other fuel sources) or from other chemical reactions or processes. Such gases often include complex chemical mixtures, and can include one or more chemical species that pose significant environmental risks and are subject to regulation by governments or other organizations. More particularly, an exhaust gas may include one or more acidic species, such as halogenated acids (e.g., HCl, HF, and HBr), sulfur dioxide (SO2), sulfur trioxide (SO3), and sulfuric acid (H2SO4), amongst other acidic species. Such species can be toxic and/or contribute to acid rain, and must therefore be treated. Exhaust gas can also include a plurality of other chemicals (e.g., arsenic and/or selenium) which can affect manufacturing process conditions by fouling catalysts (e.g., SCR catalysts) and downstream equipment.
Current methods for treating an exhaust gas and such contaminants commonly include contacting the exhaust gas with calcium hydroxide (e.g., hydrated lime or lime hydrate) particles. For example, sulfur-containing gases can be removed from a stream of exhaust gas by reacting calcium hydroxide with gaseous sulfur trioxide to form solid calcium sulfate according to the following reaction:SO3(g)+Ca(OH)2(s)Ca(SO4)(s)+H2O(g)Hydrated lime systems utilizing calcium hydroxide particles are commonly used in operations for continuously treating exhaust gases and removing acidic species.
One challenge commonly associated with these hydrated lime systems is their relative inefficiency with capturing acidic species from an exhaust gas stream. This inefficiency is due in part to the difficulty of effectively dispersing the calcium hydroxide particles quickly enough within the ducting through which the exhaust stream travels so that the particles can contact the acidic species in the exhaust gases. For example, in those hydrated lime systems that use fine calcium hydroxide particles (e.g., particles with diameters less than 8 microns) with a narrow particle size distribution, the dispersion of those particles within an exhaust ducting can be limited, and often fails to contact sulfur-containing gases at, e.g., the outer edges of the ducting in systems with short residence times. As another example, in those hydrated lime systems that use coarse particles (e.g., particles with diameters above 30 microns), the reactivity of those particles is limited because of their lower surface area to volume ratios, relative to finer particles. In addition to these particle size constraints of fine and coarse, the removal efficiency of calcium hydroxide particles can further be based on the particle surface area, particle size distribution, pore volume, and moisture content, amongst other factors. Accordingly, there exists a need for an improved composition of calcium hydroxide particles to more efficiently remove contaminants from exhaust gases.